The present invention relates to scrubbing and drying apparatus employed to clean the surface of sheet materials and more particularly to scrubbing and drying apparatus used for conditioning the surface of printed circuit boards.
Printed circuit boards are used in a multitude of electronic applications. Each printed circuit board comprises a substrate of thin flat insulating material having a conductive surface applied to one or both sides. Portions of the conductive material are removed through chemical etching processes to leave the desired pattern of interconnections on the surface of the board. Electronic components are interconnected between the conductive patterns remaining on the board such as by soldering thereto to provide the finished electronic circuit. To assure proper electrical connection of the leads of the electronic components soldered to the conductive portions of the surface and to eliminate possible conductive paths across the insulating surface of the substrate between adjacent conductive portions from deposits remaining from the etching process, the surface of the boards must be thoroughly washed, scrubbed, and dried.
Apparatus for automatically accomplishing the scrubbing and drying procedure is not new in the art. Typically, a plurality of coated rollers driven in unison are disposed transverse of a box-like enclosure in opposed parallel-spaced relationship to form a pathway through which the printed circuit boards are moved. The boards are inserted between the first pair of opposed rollers at one end and move lengthwise through the enclosure between the driven rollers to the opposite end from which they emerge. Intermediate the inlet and outlet from the scrubber/dryer enclosure, the boards are scrubbed on one or both sides by a rotating roller or brush in combination with water and then dried.
Such scrubber/dryer apparatus according to the prior art contains two major shortcomings. First, the method of positionally adjusting the scrubbing rollers is generally inaccurate in relation to the degree of accuracy required when working with such precision parts. Specifically, it is desirable to maintain the roller in a completely parallel relationship to the surface of the printed circuit board passing therethrough while retaining the capability for positionally adjusting the roller with relationship to the surface itself. The amount of pressure applied by the roller scrubbing the surface of the printed circuit board is critical and should be kept equal across the entire surface. If too little pressure is applied, the scrubbing operation is incomplete. If too much pressure is applied, the conductive surface on the printed circuit board can be damaged as by "sanding" through and/or the lifting thereof from the surface by lifting forces created in combination with heat generated by high friction.
A typical prior art technique for adjusting the position of a scrubbing roller within such apparatus is shown in FIG. 1. The view of FIG. 1 is a simplified view looking longitudinally into a portion of a scrubber/dryer unit. That is, the printed circuit board generally indicated as 10 is moving into or out of the paper as viewed. The aforementioned enclosure comprises a pair of parallel spaced sidewalls 12 having a top 14 therebetween. A pair of independently movable side members 16 are carried by the sidewalls 12 in a manner to be movable vertically as indicated by the arrows 18. The scrubber roller 20 (typically of a material sold by the 3M Company under the name Scotch-Brite) is journaled between the side members 16 for rotation by a rotating means (not shown). A so-called "Billy roller" 22 is rotatably journaled to the sidewalls 12 in parallel, opposed, spaced relationship to the scrubber roller 20. The printed circuit board 10 is supported by the Billy roller 22 as it passes beneath the scrubber roller 20. To provide for various thicknesses of printed circuit board 10 and to vary the pressure being applied to the surface of the circuit board 10 by scrubber roller 20 as it rotates in its scrubbing operation, movable side members 16 are moved vertically up or down. To maintain the constant pressure across the width of the printed circuit board 10 as previously discussed, scrubber roller 20 should be maintained in a completely parallel relationship to the Billy roller 22. To effect this simultaneous movement of movable side members 16, a pair of shafts 24 are rotatably carried by top 14 on one end and threadedly engaged into corresponding threaded holes in movable side members 16 on the opposite end. As shown, each shaft 24 extends through top 14 and has a toothed gear 26 attached thereto. One shaft 24 additionally has a knurled knob 28 on the end thereof for the gripping thereof and turning of the one shaft 24 thereby by an operator. The two toothed gears 26 are interconnected by a chain 30 such as a bicycle chain. Thus, by turning knurled knob 28, the associated shaft 24 and toothed gear 26 thereon is rotated. The chain 30 is moved in combination with the one toothed gear 26 to, in turn, rotate the second toothed gear 26 engaged therewith and, correspondingly, its shaft 24. All things being perfect, this prior art technique would probably accomplish its desired end. Unfortunately, the chances of having the threads exactly equally positionally spaced and the positioning of the links of chain 30 meshing with the teeth of toothed gears 26 at precisely the exact positional relationship are remote. Likewise, such apparatus is subject to lack of precision and cummulative tolerance motions throughout whereby the desired parallel movement objectives are not, in actuality, met.
The second major drawback is the high noise associated with the drying operation in most prior art scrubber/dryers. In order to remove the water from the surface of the printed circuit board prior to its exit from the enclosure, prior art scrubber/dryers typically employ an air knife. An air knife comprises a plenum chamber having a thin slit therein extending longitudinally traverse the path of the printed circuit board. High pressure air is forced into the plenum and out the slit forming a high energy sheet of "knife" of air which is directed in the direction opposite the movement of the printed circuit board to peel the film of water from the surface thereof in the manner of a knife. Both the motor/fan combination employed to generate the high pressure air and the high pressure air itself emanating from the slit are sources of high pitch and high volume sounds well beyond the typical employee and safety and welfare standards being imposed by various agencies such as OSHA.
Wherefore, it is the object of the present invention to provide an improved printed circuit board scrubber/dryer providing for true precision adjustment of the roller spacing while maintaining its parallel relationship to the corresponding Billy roller to form a true parallel path for the movement of the printed circuit board therethrough and incorporating an efficient drying apparatus not requiring the high pressure air knife arrangement of the prior art.